The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Many analog and digital power supplies, including average current mode power supplies, include DC to DC converters. DC to DC converters can be of different types (e.g., Buck, boost, Buck-boost, fly-back, etc.). Buck type DC to DC converters include an inductor in an output stage. An output voltage of the DC to DC converter may be regulated in many ways. For example, in average current mode power supplies, current through the inductor (inductor current) may be used to regulate the output voltage. Accordingly, sensing the inductor current is an important function of a controller for high-efficiency DC to DC converters.
The sensed inductor current can be used in a variety of applications. For example, the applications may include over-current protection, adaptive voltage positioning, and loop control for current-mode power supplies. Additionally, the sensed inductor current can be used to determine when to switch from Continuous Conduction Mode (CCM) to Discontinuous Conduction Mode (DCM). Further, the sensed inductor current can be used to calculate load current in a state space based controller, where estimated capacitor current is available.
Methods for sensing the inductor current need to be accurate, low cost, and have minimum impact on the overall efficiency of a DC to DC converter. Several methods are currently used to sense the inductor current. Each of these methods, however, has some shortcomings that make these methods undesirable for a leading-edge, high-precision, and fast-transient DC to DC converter. Some of these methods are described below.
Referring now to FIG. 1, a converter 10 uses a high-precision resistance connected in series with the inductor to sense the inductor current. The converter 10 comprises a pulse-width modulation (PWM) controller 12, a pair of series-connected switches 14 and 16, an inductor L, a high-precision sensing resistor Rsense, a capacitor Cout, and an amplifier 20. Rdc is a parasitic resistance of the inductor L.
The PWM controller 12 generates pulse-width modulated pulses that control on-off times of the switches 14 and 16. Current i flows through the inductor L and generates a voltage drop across the sensing resistor Rsense. The amplifier 20 has a gain of Av and amplifies the voltage drop across the sensing resistor Rsense. The output of the amplifier 20 is given by i*Rsense*Av. The inductor current i can be determined from the output of the amplifier 20.
This method suffers from losses in the sensing resistor Rsense, which reduces the overall efficiency of the converter 10. Additionally, this method suffers from effects of noise since the voltage drop across the sensing resistor Rsense used to sense the inductor current i is small. Further, the measurement circuits used to measure the inductor current i add a delay.
Instead of using the sensing resistor Rsense, an on-resistance (RDSon) of the switches 14 and 16, can be used to sense the inductor current i. RDSon is a resistance between a drain and a source of a switch when the switch is on. When the switch is on, RDSon of the switch is in series with the inductor L, and the inductor current i generates a voltage drop across RDSon (i.e., VDS), which can be measured to sense the inductor current i.
While this method does not affect the efficiency of the converter, this method is not very accurate since the value of RDSon varies based on temperature. Further, small signal levels of VDS pose noise problems. Additionally, measurement circuits used to measure the inductor current i add a delay.
Referring now to FIG. 2, a converter 30 uses a resistor and a capacitor connected in parallel to the inductor to sense the inductor current. The converter 30 comprises the PWM controller 12, the switches 14 and 16, the inductor L, and the capacitor Cout. Additionally, a resistor R and a capacitor C are connected across the inductor L as shown. RDC, the parasitic resistance of the inductor L, serves as the sensing resistor.
The values of R and C are chosen so that the impedance of the RC circuit formed by the resistor R and the capacitor C matches the impedance of the inductor L. In other words, the values of R and C are chosen so that the time constant of the RC circuit matches the time constant of the LR circuit formed by the inductor L and the parasitic resistance RDC. That is, the values of R and C are chosen so that R*C≈L/RDC.
When R*C≈L/RDC, the voltage across the capacitor C is linearly proportional to the inductor current i. The amplifier 40 amplifies the voltage across the capacitor C and generates an output equal to i*RDC*Av. The inductor current i can be determined from the output of the amplifier 40.
The accuracy of this approach depends on how closely the impedance of the RC circuit matches the impedance of the inductor L. Further, small voltage levels across the capacitor C pose noise problems. Particularly, the sensed inductor current may include high-frequency noise. Additionally, measurement circuits used to measure the inductor current i add a delay.